Thermoelectric devices of single phase tl2te3 and its system



y 1965 A. K. H. T. RABENAU 3,181,303

THERMOELECTRIC DEVICES 0F SINGLE PHASE Tl T8 AND ITS SYSTEM OriginalFiled July 10, 1959 2 Sheets-Sheet l TI TO Iflll I llLl I l 0 I5. 32..r-IBD' 25.3. at 1 QNaa' 1L1 LL11 ||L..| 1| uLllhlllll b D i 20 1 30 I 40DEGREES O FIG.1

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H63 PHOTON ENERGY Bi-containing 6 Bi-containing alloy solder lloy soldermvsmox. A. K. H.T. RABENAU f. AGENT United States Patent OfficeZijlfidii Patented May 4, 1965 3,181,303 j THERMQELECTRIC DEVICES F.SINGLEPHASE Tl Te, AND ITS SYSTEM Albrecht Karl HeinrichTheodor Rabenau,Aachen, Germany, assignor to NorthjAmerican Philips Company, Inc NewYork, 'N.Y., a corporation of Delaware Original application July 10,1959, Ser. No; 826,341, new Patent No. 3,096,151, dated July 2, 1963.Divided and this application Dec; 1, 1961, Ser. No. 171,545 Claimspriority, application Germany, July 23, 1958, a N 15,384; June 11, 1959,N 16,780

6 Claims. (Cl. 62-3) This invention relates'to semi-conductor devices,and in particular to thermo-electric devices employing the Peltiercooling effect. vThis is a division ofapplication, Serial No. 826,341,filed July 10, 1-959, and now Patent No. 3,096,151.

Many known semi-conductor elements and compounds are employed in thesemi-conductor field; and, especially in the transistor and diode artseveral have become widely accepted because of their advantageouscombination of properties required for this application. However, thereare other fields of application of semi-conductor devices in which theknown semi-conductor materials, while capable of performing the requiredfunctions to a certain extent, still do not possess the combination ofproperties required for this application to a sufiiciently high extentto make them competitive with other devices operating on differentprinciples and now in general use in this field. This is especially truefor Peltier cooling devices, sometimes referred to as Peltier heat pumpsor semiconductor refrigerators, for which the best of the knownsemiconductors, namely Bi Te or mixed crystals thereof, do not yetexhibit the required-low value of thermal conductivity combined with asufficiently high value of thermoelectric power to equal in cost andefiiciency the compression-type cooling devices generally used inrefrigerators.

One object of the present invention is to provide a new semiconductormaterial having a-very low thermal conductivity and a highthermoelectric power so that it is particularly suitable for use inthermoelectric devices employing the Peltier cooling effect.

The concept underlying the invention is that a material possessing therequired properties exists in the thalliumtellurium system. Tins systemTl/Te was examined several times before 1940, and, according to Gmelin,Handbuch der' anorganischen Chemie, 8th edition, the compounds Tl Te, TiTe TlTe, Tl Te and Tl Te were found to exist. However, in Zeitschriftfiir anorganische allgemeine Chemie, 260, 110 (1949), the existence ofseveral of these compound is denied,,and an extensive X-ray diffractionanalysis of the system Tl/Tc is described, from which it was ascertainedthat samples of compositions ranging from TlTe to TlTe which were heatedfor 24 hours at temperatures varying from 200 C. 400 C. in accordancewith the proportion of tellurium, showed only a single phase belongingto the compound TlTe.

The new semi-conductor material of the invention is the compound Tl Teg,and mixed crystals of this compound, in which the structure of thecompound is retained while part of the thallium or the tellurium or bothis replaced by other suitable elements. It has been found that thiscompound Tl Te exists and possesses excellent properties as asemi-conductor. For example,

it possesses a high thermo-electric power, a low thermal conductivity,ahigh temperature coefiicient of resistance, and a high sensitivity toradiation. These properties render this compound and its isomorphousmixed crystals suitable for use in semi-conductor devices for which atleast one of these properties is of importance, for example, fortemperaturedependent resistors. However, the material in accordance withthe invention is especially suitable for use in thermoelectric devices,for example, in a Peltier refrigerator, in which, according to the invemtion, at least one leg is made of the new material. Preferably such adevice in accordance with the invention has at least two successive legseach made of the new semiconductor material but having opposite types ofconductivity. v i

The inventionwill now be described more fully with reference tothe'accompanying drawings, in which:

FIGS. 1a and 1b are line spectrums obtained by a powder difi'ractionanalysis of two somewhat similar materials of which only the secondmaterial is the new com pound of the invention; a

FIG. 2 is a graphical representation of the temperature dependence ofconductivity of two sample materials of Example I A sample of thalliumof a purity higher than 99.5% was subjected to a milling machineoperation in an argon atmosphere until its surface appeared bright. Thesample then weighed 47.34 g'rns. Then, the sample was introduced in anargon atmosphere into a glass tube closed at one end. To obtain thestoichiometric composition of of the compound Tl Te 44.34 gms. oftwice-distilled teilurium was added to the tube. Next, the tube wasevacuated, sealed tight and then heated in an electric furnace toa'temperature of 450 C., and held at this temperature'for about 20minutes. During this heating step, the tube was shaken to'intimately mixits molten contents. Next, the furnace was cooled to 245 C. and

Afterwards,

maintained at thistemperature for 5 days the tube was removed from thefurnace and. cooled further in air. -Gne quarter of the tube contentswas removed, and this quarter will be referred to hereinafter assample 1. The remainder was again sealed back in the tube, which wasthen placed in a furnace and heated at 201) C. for 5 additional days.The tube was then removed, a part of its contents removed, which partwill be referred to hereinafter as sample 2, and the remaining partresealed in the tube and reheated to 200 C. and maintained thereat for12 additional days. The remainder of the tube contents will be referredto hereinafter as sample 3. r

The three samples were then analyzedby means of X-ray powder'diifractometry according to the asymmetrical method of St'raumanis. TheX-ray apparatus 7 used was a Muller Mikro 101 with stabilized voltage.The analyzing radiation was Cu K: filtered by nickel; the radius of thefilm chamber was 57.3 mms.; the exposure time was 3 hours at an X-raytube voltage of kv. and tube current of 25 ma. FIG. 1 shows the linespectrums obtained on the film, with FIG. 1a obtained from sample 1 andFIG. 1b from sample 3. The estimated values of the intensities areplotted linearly in arbitrary units along the vertical axis while theangle of deflection 0 of the diffracted beam is plotted horizontally.From FIG. 1, it will be seen that the X-ray diagram of sample 3 (FIG.1b) is clearly distinguished from that of sample 1 (FIG. 1a). The X-raydiagram of the sample 1 is found in qualitatively equal form for thecompound TlTe, which was produced separately in the above-mentionedmanner with a corresponding composition of the components in order toprovide a comparison with samples 2 and 3. The sequence of lines in FIG.1b derived from sample 3 is characteristic of the compound Tl Te or ofmixed crystals thereof. The lines indicated by arrows in FIG. 1b can beused to detect the presence of the compound in accordance with theinvention or mixed crystals thereof from an X-ray diagram. The sample 2showed no radiographical difference with the sample 3, so that thesample 2 also consisted of the compound in accordance with theinvention, whereas in the sample 1 the compound in accordance with theinvention had not been formed, the reason for which will be explainedlater.

As a further means of distinguishing the compound Tl Te from othersimilar compositions, the samples were studied under a microscope. Tothis end, a portion of the sample was ground, polished and etched. Whenmagnified a hundred times, the ground faces showed that the sample 1consisted of a primary crystallization embedded in a eutectic, so thatthere were two phases present. In contradistinction thereto, the sample3 in accordance with the invention showed a-single phase of a uniformcrystal structure, some of the crystals having sizes up to a few tenthsof a millimeter. Since the stoichiometrical composition was employed,this single phase had to be Tl Te In the sample 2 in accordance with theinvention, this latter structure had not yet been formed so completely.

Both methods of examination show unambiguously that the samples 2 and 3are the compound Tl Te in accordance with the invention, whereas thesample 1 was not converted into this compound, apparently since it washeated above the decomposition temperature of the compound of theinvention.

Subsequently, measurements of the electrical conductivity, thethermoelectric power or thermo-E.M.F. and the Hall constant were made onthe various samples in a conventional manner. For this purpose, smallersamples of about 3 by .1 by 3 cubic mms. were cut from thepolycrystalline solid material. Ohmic contacts were applied to thesamples by alloying thereto bismuth-plated copper wires. This was doneby local heating for a short period of time. In the Hall constantmeasurement, a magnetic field strength of 54-00 Gauss was used. Theresults at two ditferent measuring temperatures, namely roomtemperature, 20 C., and l80 C. are shown in the following table.

From this table, it will be seen that sample 1 exhibits metallicbehaviour in its electrical properties, while the results for samples 2and 3 in accordance with the invention are characteristic ofsemiconductors. Furthermore, the table shows that samples 2 and 3 inaccordance with the invention have a particularly high thermo-BMF.

Furthermore the temperaturedependence of the electric conductivity wasmeasured on samples 2 and 3 in accordance with the invention. FIG. 2shows the curves resulting from these measurements, the logarithm of theconductivity in ohnr emf being plotted as the ordinate and as theabscissa, where T is the temperature in K. The curve 1 is for the sample2 and the curve 2 for the sample 3. From the slope of the straightportions of these curves, activation energies of 0.65 ev. and 0.63 ev.are found for the samples 2 and 3, respectively. In order to identifythis measured activation energy, the band spacing was determinedoptically in the following manner. The measurement was made on apowdered sample by means of remission measurement. This method ofmeasuring is based on a comparison of the spectral intensities I and Iwhich are reflected by the sample to be examined and by a layer of MgOas a standard. FIG. 3 shows the measurements,

n log I (measure of the absorption) being plotted as the ordinateagainst the photon energy in electronvolts as the abscissa. The curves 3and 4 show the measurements made at room temperature and at C.,respectively. In the range of the absorption edge, the curves show asubstantially rectilinear variation. From the curve 3, an optical bandspacing of about 0.65 ev. is found for the sample at 20 C. Thesubstantial equality of the optically measured band spacing of Tl Te andof the activation energy shown in FIG. 2 makes it highly probable thatthe straight portions of the curves 1 and 2 of FIG. 2 show the ranges ofintrinsic conduction of the samples in accordance with the invention.From the measurements given, and the supposition that the conventionalformulas for semi-conductors may also be used for this case, andassuming an elfective mass equal to the electron mass, there is found atroom temperature for the carrier mobility a sum total of the electronand hole mobilities between about and 10,000

vsec.

With a smaller effective mass, still higher carrier mobilities arefound, while smaller carrier mobilities require an effective mass largerthan 1. In any case, it will be seen from these measurements that thesemi-conductor compound in accordance with the invention has particularproperties with respect to these quantities also.

In order further to examine the suitability of the compound inaccordance with the invention for use in thermoelectric devices, thethermal conductivity of the sample 3 was determined in the usual manner.It was about 6 10- w./cm. degree. This value is materially lower thanthat of other semiconductors which might be considered for use inthermo-electric devices, and it is indeed surprising that the compoundin accordance with the invention at the same time has a thermo-E.M.F.which is materially higher than the values measured on the othercompounds of this kind. These excellent properties also prove theparticular suitability of the semiconductor compound in accordance withthe invention (and, as will be shown hereinafter, of the mixed crystals'of this compound) for use in thermoelectric devices, for example, inPeltier refrigerating elements.

In order to ascertain the conditions of formation, a number of testswere made on samples which were produced by fusion of the stoichiometriccomposition at different temperatures and, with the use of differentheatingperiods. The samples were identified by means of X-ray analysis.The results wjere as follows:

(a) In a sample heatedfor 20 minutes at 450 C. and subsequentlyquenched. at room temperature, no Tl Te could be detected.

i (b) In a sample heated at 245 C. for 3 days after fusion, no Tl Tecould be detected.

(c) a sample, heatedrfor 3 days at 210 C. after fusion, was converted tothe compound TlzTeg in accordance with the invention, and this was alsothe case for a sample heated for 3 days at 200 C. after fusion.

(d) A sample, heated-at 180 C. for 3 days after fusion, was convertedalmost completely into the compound in accordance with the invention,whereas in a sample annealed for 3'days at 150 C., the conversion couldhardly be detected, since at this temperature the duration of treatmentwas too short.

By these temperature treatments, an accurate value of the decompositiontemperature could not be determined, since it is very diflicult inpractice to keep the annealing temperature exactly at a predeterminedtemperature for so long a period of time. A more accurate indication onthe decomposition temperature was found by means of the followingexperiments.

For an accurate determination of the decomposition temperature ofTlzTeg, the following heat-treatments were carried out with samples ofdiflierent composition. The starting materialiconsisted of three sampleshaving compositions between TlTe and Tl Te These samples were heated at220 C. for 600 hours; after the experiments, they were thermally stable;they contained 55, 59 and 59.3 atomic percent of Te, respectively. Withsamples of such composition, observation of a possible partial meltingof the sample when heated to a temperature in the proximity of thedecomposition temperature permits of ascertaining with certainty whetherthe decomposition temperature has been exceeded, since in this event thesample decomposes into TlTe and a liquid phase. The samples werecompressed to form pellets, which were sealed in glass tubes. They wereheated in a Htippler thermostat having a filling of silicone oil. Thetemperature constancy of the thermostat was approximately 02 C. Theabsolute value of the temperature was read from a calibrated mercurythermometer. The following heattreatments were carried out:

1st treatment: The three samples were heated at 225 C. for 100 hours.Observation by microscope showed no sign of partial melting. Hence, thistemperature was certainly lower than the decomposition temperature.

2nd treatment: The three samples were heated at 236 C. for 100 hours.Observationby microscope showed no sign of partial melting. Therefore,this temperature also was certainly lower than the decompositiontemperature.

3rd treatment: The three samples were heated at 237 C. for 100 hours. Atthis temperature, partial melting could not be ascertained withcertainty.

4th treatment: The three samples were heated at 238 C. for 160 hours. Atthis temperature, in all three samples visual observation revealedpartial melting. Hence, this temperature was at least equal to thedecomposition temperature.

Thus, these'experiments show that the decomposition temperature forthecompound Tl Te is just below 238 C. and is approximately 237 C.

From further tests, it was also found that preparations produced in themanner described with respect to sample 1 and the compositions of whichranged from TlTe to TlTe showed metallic behavior when they were heatedabove about 238 C., the composition T1Te not distinguishing itself byany discontinuity (thermal arrest) of its properties (evenradiographically). If, however, this composition is heated for aprolonged period of time below the decomposition temperature, thecompound Tl Te in accordance with the invention was produced having aparticular structure as shown by the Debye powder analysis andexhibiting an abrupt variation .of

its properties, which are typical for a semiconductor."

To sum up, it is therefore evident that, in order to carry out themanufacture of the new material of the invention, it must'be prepared ata temperature below the decomposition temperature of the desiredcompound in the solid state. Tothis end, the components or the compoundssupplying these components in a suitable mixing ratio are heated beforeor after being shaped into the form desired for the body, at atemperature below the decomposition temperature of the desired compoundin the solid state for a sufficiently long period of time until thedesired compound is produced. The compound Tl Te or the isomorphousmixed crystals thereof, decompose in the solid state above acomparatively low temperature, so that they can be produced only belowthe decomposition temperature associated with the desired compound, and,as noted, the conversion requires a comparatively long period of time.This explains why hitherto this compound has not been found in spite ofvarious researches into the system Ti/Te, since in the suitable range ofcompositions the temperature was chosen above the decompositiontemperature and possibly the heating period was chosen too short. Thedecomposition temperature for the com pound Tl Te lies below about 238C. It should be noted that the decomposition temperature of theisomorphous mixed crystals can differ from the decomposition temperatureof the compound. Heating preferably is eifected below about 238 C. andeven below 230 C. ()bviously the reaction velocity is higher at a highertemperature. Below C, the required heating period becomes prohibitivclylarge for practical purposes, so that heating is preferably effectedabove l50 C. The temperature range between about C. and about 238 C. isparticularly suitable. Furthermore, heating is preferably effected in anon-oxydizing atmosphere, such as hydrogen, argon or a vacuum.

The components concerned or the compounds supplying these components orboth are preferably heated as a finely-divided powdered mixture, beforeor after shaping of the mixture into the form desired for the body, forexample by compression. In practice, particularly good results are alsoobtained by melting together the components concerned or the compoundssupplying these components or both and subsequently heating them in atemperature range below the decomposition temperature of the desiredcompound. Preferably, this latter process is carried out by cooling themelt to a temperature below the decomposition temperature, whereuponthey are heated at this temperature in order to form the required coirpound. Obviously, there is a large variety of manners in which such amethod can be carried out. Thus, the fused mixture may be powdered andsubsequently heated for conversion into the desired compound, if desiredafter it has been shaped into the form desired for the body. If asintered body is to be made, the powdered mixture is preferablycompressed at a temperature below the decomposition temperature of thedesired compound and simultaneously or subsequenti y heated forconversion into .thedesired compound. 7

After the semi-conductor body has been heated for conversion into thedesired compound, the body will generally have to be subjected, at leastlocally, to a further heat treatment, for example, for providing it withelectrical contacts. According to a further feature of the invention,such a heat treatment is preferably effected below the decompositiontemperature, or this further heat treatment is performed for such ashort period of time above the decomposition temperature that theduration of the heat treatment is too short to give rise todecomposition of the compound. As a further alternative, the furtherheat treatment can be effected above the decomposition temperature,after which the body is heated to a temperature below the decompositiontemperature in order to regenerate the desired compound. For manyapplications, the body must be provided with at least one ohmic contact.According to still a further feature of the invention, use is preferablymade of a contact consisting of an alloy containing bismuth which isalloyed to the body. Obviously, other contact materials may also beused.

An example of a body made by a powder technique is now described:

Example ll Since thallium cannot be powdered, TlTe was used as thestarting material. This was ground to form a powder together with anamount of tellurium stoichiometrically required to produce the compoundTl Te the resulting powder being compressed to form bodies under apressure of about 1 ton/cm. The shape and dimensions of the bodies soformed were: pills having a diameter of 22 mms. and a height of from 8to 10 mms., and rods of 5 by 5 by mmfi. These were heated in vacuum orin a protective gas atmosphere, e.g., hydrogen and argon, at 200 C.After 8 days at this temperature, the samples were removed from thefurnace, and it was found from X-ray analysis that the compound inaccordance with the invention had been formed. Furthermore, the measuredvalues of the electric and thermal properties were of the same order ofmagnitude as described with respect to sample 3 in Example I.

The invention relates not only to the compound Tl' Te but also toisomorphous mixed crystals resulting from this compound. The formationof mixed crystals is a known, generally-used process in semiconductortechnology for the purpose of conversion of a compound which in acertain respect is particularly suitable for a special purpose into amixed crystal which, in addition, has other useful properties for thisspecial purpose. Thus, for example, for use in Peltier refrigerators,part of the thallium of the compound may be replaced by gallium orindium, and part of the tellurium by sulphur or selenium in order toreduce the thermal conductivity. Obviously, the formation of mixedcrystals is not restricted to these replacements; the thallium mightalso be replaced with up to for instance 25 atomic percent of bismuth,lead or mercury. Furthermore the term compound obviously is not to beunderstood to mean the precisely stoichiometric compound Tl Te only,but, in the manner usual in semiconductor technology, also includes anydeviations from the precise stoichiometric composition which may occurwithin the phase limits and furthermore the additional introduction ofactive imperfections, more particularly, impurity atoms. Thesedeviations from the stoichiometric composition, for example by theincorporation of a large relative quantity of thallium or tellurium, andthe additional doping with impurity atoms such, for example, as, on theone hand, copper or silver and, on the other hand, a halogen such asiodine, may be used to modify the conductivity type or the conductivityor both of the body. Examples illustrating this follow.

Example III In order to examine the formation of mixed crystals, asample having a composition according to the formula Tl A Te whereA=indium and x=0.05, was annealed at various temperatures. The procedurewas as follows: A piece of thallium, which was machine milled bright inan argon atmosphere and weighed 25.463 gms., was inserted, in an argonatmosphere, in a glass tube closed at one end. 24.442 gms. of telluriumand 0.367 gms. of indium were added to the tube. Then the tube wasevacuated (weak vacuum) and sealed. The purity of the thallium andtellurium was the same as in Example I and the indium was spectrallypure. The tube was heated in a furnace for half an hour to 500 C.,shaken for homegenization, and subsequently cooled in the furnace toabout C. and then removed. The duration of the cooling was about 3hours. The sample preparation was taken from the glass tube, a part wasremoved and the remainder again sealed in vacuum. This latter part washeated to 350 C. so that it was completely molten and then cooled inair. Next, it was heated at 200 C. in the furnace and left at thistemperature for 100 hours. Then the tube was taken from the furnace andcooled in air.

The sample cooled down from 500 C. showed substantially the same linespectrum as the Sample 1 of Example I. See FIG. 1a. The sample heated at200 C. showed only the lines of the compound Tl Te in accordance withthe invention. The thermo-E.M.F. and the termal conductivity of thesample in accordance with the invention were measured. The thermo-E.M.F.was +380 microvolts/ degree and the thermal conductivity was about 30%lower than the value given in Example I for the sample 3.

Example IV This time a sample having a composition according to theformula Tl Te B where B is selenium and y=0.05 was annealed at varioustemperatures. The procedure was exactly the same as descrbed in ExampleIII; however, the additions were: thallium 22.317 gms., tellurium 20.551gms. and selenium 0.215 gms. The purity of the elements thallium andtellurium was the same as in Example I. The selenium used had beendistilled twice.

The sample which'was cooled after being heated to 500 C. showed aboutthe same line spectrum as the sample 1 of Example I (FIG. 1a) With,however, some additional lines. The sample heated at 200 C.substantially showed only the lines of the compound Tl Te in accordancewith the invention (FIG. 1b). The thermo- E.M.F. and the thermalconductivity of the mixed crystal sample in accordance with theinvention were measured. The thermo-E.M F. was +600 microvolts/ degreeat room temperature and the thermal conductivity was about 30% lowerthan the value given in Example I.

Example V This time the mixed crystal constituted a deviation from thestoichiometric composition. A preparation containing a large relativequantity of tellurium and comprising 60.1 atomic percent of telluriumand 39.8 atomic percent of thallium was produced in a manner similar tothat described in Example III and heated at 200 C. The amounts by weightwere 4.904 gms. of thallium and 4.624 gms. of tellurium.

After heating, X-ray examination showed substantially the same linespectrum as for the stoichiometric composition Tl Te (FIG. 1b). Thethermo-E.M.F. was +435 microvolts/degree.

Example VI Similarly to Example V, a preparation containing a smallrelative quantity of tellurium and comprising 59.8 atomic percent oftellurium and 40.1 atomic percent of thallium was treated in the samemanner as described in Example III and heated at 200 C. The amounts byweight were 6.133 gms. of thallium and 5.707 gms. of tellurium.

After annealing, X-ray examination showed substantially the same linespectrum as the stoichiometric composition Tl Te The thermo-E.M.F. was+270 microvolts/degree.

9 Example VII In a-further examination of the'formation of mixedcrystals, 1.5 'mol. percent. of PbTe was added; to a sam ple of the'stoichiometric composition- Tl Te in accordance with the invention.Then the aggregate was ground,

pressed to form pills and subsequently-heated at- 210 C.

for days. X-ray examination again showed the line spectrum of FIG. 1b.The thermo-E.M.Fl was650 microvolts/degre'e and the thermal conductivitywas about 20% lower than than given in Example 1 for sample. 3.

It should be noted that: in the mixed crystals in accordance with theinvention as described, for example, in Examples III to VII, X-rayexamination may show small deviations in the line intensities and linespacings. However, the characteristic line sequence is alwaysmaintained. In order to identify the compound in accordance with theinvention or" mixed crystals thereof, obviously in addition to orinstead of the X-ray examination, use may be made of examinations of theelectrical and/or thermal properties, for, as has been describedhereinbefore, in the formation of the compound in'accordance with theinvention or the mixed crystals thereof these properties show abruptchanges.

Example VIII This example relates to doping with impurity atoms.

To a powdered preparation consisting of the compound Tl Te in accordancewith the invention was added about 0.5 atomic percent of iodine. Theaggregate was sealed in a vessel in argon under a pressure of from 10mms. to 100 mms. and subsequently heated at 200 C. for 3 days. Then thesample was taken out and compressed to form apill and subsequently againheated at 200 C. for 3 days. Measurements showed a thermo-E.M.F. ofabout +700microvolts/degree, and an electrical conductivity about 3times that of sample 3 (Example I). From this example and also fromfurther similar doping tests, it was found surprisingly that in thecompound in accordance with the invention and the i'somorphous mixedcrystals thereof the electron conductivity can be materially increasedwithout a substantial reduction of the thermo- E.M.F. Thus, a similarbehaviour Was found when doping with copper, for example, with 0.5atomic percent. By using similar doping techniques known in the art, theconductivity of the samples may be controlled.*

Now, an application of the semiconductor compound in accordance with theinvention will be described with reference to FIG. 4. This FIG. 4 is anelevational view of a constructionalunit of a Peltier refrigerator. Thephysical construction of such a unit is Well-known. It comprises twolegs 6 and 7. These two legs are not soldered to one another directly.In order to improve the heat dissipation, at the upper side of thedevice a length 8 of a substance of high thermal and electricalconductivity, for example copper or silver, is inserted between thelegs. In a device in accordance with the invention, at least one of thelegs 6 and 7 contains or comprises at least one of the semiconductors inaccordance with the invention; preferably both legs consist of asemiconductor material in accordance with the invention, theconductivity types of the two legs being opposite. The current issupplied and taken oif at the lower side through'contacts 9 and lit in amanner such that at the upper side the Peltier heat is absorbed and atthe lower side the Peltier heat is produced. According to the invention,the part 8 and the contacts 9 and 10 are soldered to the legs by meansof an alloy 11 containing bismuth.

It will be appreciated that the invention is not restricted to thisthermoelectric application in Peltier refrigerating elements or to theparticular embodiment of such a device, and that the semiconductors inaccordance with the invention can be used to great advantage in otherthermoelectric devices, for example thermoelement and thermoelectricheat pumps. Similarly, the semiconductor bodies in accordance with theinvention can be used in other semito the above-described mixed crystalbodies. Thus, mixed crystals may be made with the use ofdifferentelements, and the formation of mixed crystalsis not restricted to thevalues given by way of example. The conductivity type or theconductivity or both can be influenced in the ways generally used inthis fieldof technology.

While I have described my invention in connection with specificembodiments and applications, other. modifications thereof will bereadily apparent to those skilled in this art without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A thermoelectric device comprising a body consisting essentially of asemiconductor material comprising a single phase containing thallium andtellurium having a crystal structure isomorphous with that of Tl Te andexhibiting a characteristic X-ray powder diifraction pattern containingsubstantially the line sequence indicated by the arrows in the graph ofFIG. 1b of the accompanying drawing, wherein the abscissa is in degrees0 and the ordinate indicates relative line intensity, said single phasepossessing a low thermal conductivity, a high thermoelectric power, andan electrical conductivity in the semiconductor range, and means coupledto the body for passing current through the body.

2. A device as set'forth in claim 1 wherein the device I includes twobodies of said semiconductor material, one 30 being of one typeconductivity and the other being of the opposite type conductivity, saidbodies being coupled together by means of a common conductive member.

3. A semiconductor device comprising a body consisting essentially of asemiconductor material comprising a single phase containing thallium andtellurium having a crystal structure isomorphous with that of Tl Te andexhibiting a characteristic X-ray powder ditfraction pattern containingsubstantially the line sequence indicated by the arrows in the graph ofFIG. 1b of the accompanying drawing, wherein the abscissa is in degrees0 and the ordinate indicates relative line intensity, said single phasepossessing a low thermal conductivity, a high thermoelectric power, andan electrical conductivity in the semiconductor range, and electricalcontacts to said body.

4. A device as set forth in claim 3 wherein the device includes an ohmiccontact to said body, said ohmic contact comprising a bismuth-containingsubstance surface alloyed to said body.

5. A new semiconductor system comprising a single phase containingthallium, tellurium, and an element selected from the group consistingof gallium, indium,

sulphur and selenium and having a crystal structure isomorphous withthat of Tl Te and exhibiting a characteristic X-ray powder diifractionpattern containing substantially the line sequence indicated by thearrows in the graph of FIG. lb of the accompanying drawing, wherein theabscissa is in degrees 0 and the ordinate indicates relative lineintensity, said single phase possessing a low thermal conductivity, ahigh thermoelectric power at room temperature, and an electricalconductivity-in the semiconductor range.

6. A new semiconductor system comprising a single phase containingthallium and tellurium and including an active impurity substanceselected from the group consisting of halogen, copper and silver, andhaving a crystal wherein the abscissa is in degrees 0 and the ordinateindicatesrelative line intensity, said single phase possessing a lowthermal conductivity, a high thermoelectric power at room temperature,and an electrical conductivity in the semiconductor range.

(References on following page) References Cited by the Examiner UNITEDSTATES PATENTS 12 OTHER REFERENCES Hoffman, Lexikon der AnorganischeVerbindungen,

Faus 136 5 Band 1, 1 Halfte, No. 1-31, page 730.

Fuuer' Mellor Comprehensive Treatise on Inorganic and Conrad TheoreticalChemistry, Longrnans, Green and 00., N. Y., Lindenblad 13 5 1931, 11 PWernick. JOHN H. MACK, Primary Examiner.

1. A THERMOELECTRIC DEVICE COMPRISING A BODY CONSISTING ESSENTIALLY OF ASEMICONDUCTOR MATERIAL COMPRISING A SINGLE PHASE CONTAINING THALLIUM ANDTELLURIUM HAVING A CRYSTAL STRUCTURE ISOMORPHOUS WITH THAT OF TL2TE3 ANDEXHIBITING A CHARACTERISTIC X-RAY POWDER DIFFRACTION PATTERN CONTAININGSUBSTANTIALLY THE LINE SEQUENCE INDICATED BY THE ARROWS IN THE GRAPH OFFIG. 1B OF THE ACCOMPANY-